Somewhere in the imagination, at an intersection of the idealized Golden Age and mankind's descent into manifest imperfection, existed the island civilization of Atlantis. This realm of divine origin was ruled from a splendid metropolis in the distant ocean. Its empire, described by a philosopher as "larger than Libya and Asia combined," enjoyed prosperity and great power.

In time, driven by overweening ambition, a common theme in antiquity and not unheard of today, Atlantis set out to conquer lands of the Mediterranean. But in a terrible day and night of floods and earthquakes, Atlantis was swallowed by the sea, sinking into legend.

The story endures as a classic in the genre of lost worlds long vanished, the ruins and treasures of which are surely somewhere out there yet to be found. Legends, though, are often mirages, forever shimmering out of reach, yet exerting an attractive power beyond reason.

Sometimes the pursuit of legends leads to unforeseen knowledge.

In the 12th century A.D., the legend of Prester John, a rich and powerful Christian monarch somewhere in Asia, drew intrepid seekers, eventually including Marco Polo, who opened Western eyes to the wonders of the East. When no one found Prester John in Asia, the legend did not go away; its locale shifted to Africa.

The golden city of El Dorado eluded hellbent adventurers, whose frustrated quest nonetheless put much of South America on the map.

The fabled Seven Cities of Cibola, castles in the air that proved to be nothing more than humble Indian pueblos, drew Europeans across tortured miles and years of discovery in what is now the Southwestern United States.

The tale of the lost continent has sent respected classical scholars to their texts for corroboration that Atlantis was more than fantasy. Archaeologists, geologists and divers have plumbed ocean depths where the island supposedly sank out of sight thousands of years ago. Not a scrap of compelling evidence supporting the legend has ever turned up.

Such a negative discovery might be conclusive enough for most legends to pass from rock-hard belief to literary artifacts of prescientific cultures living in a world of limited horizons and boundless mystery. But true believers, complaining that scientists have got it all wrong, continue the search.

Generations of adventurers, writers, mystics and cranks have satisfied themselves of the legend's reality. Their "solutions" fill more than 2,000 books and countless articles. The lost continent also inspired works by authors as diverse as Francis Bacon and Arthur Conan Doyle, and Hollywood has weighed in with any number of forgettable movies.

Richard Ellis, author of "Imagining Atlantis," thinks the legend is fantasy. "Atlantis lives on in people's minds largely because you cannot prove it doesn't exist," he said recently. "You can't search every inch of the ocean bottom, and so the hope remains alive and the promise of finding treasures in sunken palaces."

The sole source of the Atlantis story is by no means obscure. In two dialogues, the "Critias" and the "Timaeus," Plato in the fourth century B.C. described a resplendent island empire in the Atlantic Ocean beyond the Pillars of Hercules (the Strait of Gibraltar). "This dynasty, gathering its whole power together," Plato wrote, "attempted to enslave, at a single stroke, your country and ours."

Even after disbelief in ancient gods undercut literal acceptance of the legend, medieval maps were sprinkled with imaginary islands in the Atlantic, including Antillia. Some experts suspect this preserves in garbled form the name of Atlantis and a lingering belief that its remnants may still exist. The maps encouraged navigators in their quests, among them Columbus.

The 20th century was hard on Atlantis dreams. Detailed mapping of the sea floor and the new theory of plate tectonics made it clear, geophysicists say, that land masses resembling Atlantis never existed in the Atlantic.

Undeterred, ardent believers went looking elsewhere: in Scandinavia, the Bahamas and the Aegean Sea. Huge blocks of stone submerged off Cuba were recently proclaimed possible ruins of the lost empire.

A more plausible hypothesis, some scholars think, places Atlantis at Crete. The accomplished Minoan civilization there collapsed in the middle of the second millennium B.C., presumably destroyed by a volcanic eruption on nearby Thera, modern Santorini.

Was this in Plato's mind? Or he might have been inspired by an event in his own time, the earthquake in 373 B.C. that brought the Greek city of Helike, as ancient writers said, crashing into the sea.

The unknown fires the imagination. Whether the starry night or extraterrestrial beings, the mystery of life itself or life after death or any of the uncertain boundaries between reality and resolute yearning, it is unknowns that populate history with gods and heroes, monsters of the deep and chimeric islands, lost paradises and the elusive El Dorado at the end of greed's rainbow, not to mention Martians.

Some mysteries will be solved, but never all of them. As for Atlantis, another Greek philosopher delivered the verdict that has yet to be contradicted.

As noted by the British classicist J. V. Luce, Aristotle considered Atlantis a poetic fiction invented by Plato as a warning of the fate that befalls the arrogant and decadent. Plato placed Atlantis beyond the then known world and sank it to the ocean floor to preserve the power of the mystery.

"The man who dreamed it up made it vanish" was Aristotle's solution to the mystery of Atlantis.